The present invention relates generally to halftone screening processes and, more particularly, to a system and method to improve a halftone by combining halftoning with algorithmic processing.
Halftone screening transforms a continuous tone image into a binary image that is to be rendered and perceived by an observer as the original continuous tone image. Halftone screening processes typically apply a halftone screens to a continuous tone image. The result is binary image that appears to be made up of patterns or groups of individual black and white printer device dots. Each pattern or group has a proportion and arrangement of black and white dots so as to render, from a distance, an impression of a selected level of gray. Thus, when a halftone image is observed from a typical viewing distance, it appears as an original, continuous tone image. Currently, halftone screening is used in printing devices such as conventional printing presses, laser printers, dot matrix printers, and inkjet printers; and the like.
Halftoning is necessary because printing devices are not capable of producing all of the shades or colors often contained in continuous tone images. For example, a laser printer may have only one color of ink; typically, black. There are no grays. Halftoning permits the appearance of a number of shades of gray.
Halftone screens are created using screen frequencies, typically measured in lines per unit of length, such as lines per inch (lpi). Thus, a screen frequency is often represented by a grid. Each square in the grid then represents a halftone cell capable of holding a halftone dot pattern. Higher screen frequencies produce finer halftone screens, while lower screen frequencies produce coarser halftone screens. Further, multiple screen frequencies are represented by multiple grid or halftone screens.
To convert a continuous tone image into a halftone image, a halftone screen or grid is typically superimposed on the continuous tone image. Each halftone cell in the halftone screen or grid is then assigned a different sized dot to represent the continuous tone image data for that halftone cell. Again, when all of the dots are viewed together at a normal viewing distance, the dots appear as the original continuous tone image.
The size of the halftone cells is determined by the interaction of the selected screen frequency with the resolution of a printer's device resolution. The word “printer” refers to any mechanism that makes marks on a physical substrate. A printer creates an electronic version of the halftone screen, while screening software applies an electronic dot pattern to the electronic image. The image recorder resolution setting reflects the image recorder's ability to place device dots close together.
For example, in a typical laser printer, rollers pull a sheet of paper from a paper tray and through a “charge roller,” which gives the paper an electrostatic charge. Simultaneously, a printing drum is given an opposite charge. The surface of the drum is then scanned by a laser in accordance with the image recorder, discharging portions of the drum surface, and leaving only those points corresponding to the desired text and/or image with a charge. This charge is then used to adhere toner to the drum surface. The paper and the drum are then brought into contact, the differing charges attracting the toner to the paper. The paper then travels between “fusing rollers” which heat the paper and melt the toner, fusing the toner to the paper.
Thus, the closer together the image recorder can place the spots, the higher the image recorder resolution. Further, device dots composing the grid are commonly referred to as “printer dots” and image recorder resolution is measured in dots per inch (dpi), and may also be represented by a grid.
When the halftone grid is laid over the resolution grid, each halftone cell is filled with device dots. Groups of device dots form halftone dots. Thus, each of the halftone cells in the previous example is comprised of many device dots that are created by the image recorder, forming the halftone dots. Each of these device dots created by the laser is selectively turned on, producing a final output, e.g., gray scale, or turned off, producing no output or white.
The combination of device dots within a halftone cell produces a halftone dot of a specific size and shape. For example, if the halftone dot needs to be bigger, the laser turns on more device dots. Similarly, if the halftone dot needs to be smaller, the laser turns on fewer imagesetter spots. To create different shapes, the image recorder turns the imagesetter spots on in different sequences. Each sequence is determined by a mathematical equation referred to as a spot function or, more commonly, by a sequence of numbers referred to as a threshold array. Different spot functions and array sequences exist for each dot shape. Common shapes include round, diamond, line, square and elliptical.
Halftone names can be confusing. For example, there are two types of square shaped halftones. In one of these, the halftone dots are shaped like squares all the way through the tint or grey scale. In the other, the halftone dots start out shaped like circles, grow to square shapes in the midtones, and then become circular again. In addition, different manufacturers of printing devices use different spot functions to create halftone dots. Thus, not every manufacturer's round or square dots, for example, grow in exactly the same way.
One print standard, commonly referred to as PostScript, Adobe Systems, has emerged which includes a system for handling gray levels. PostScript has a number of ways of defining screening patterns that are built into the language and also provides for proprietary methods. PostScript requires 256 levels of gray to properly reproduce a continuous tone image. Because of this requirement, manufacturers have adopted 256 gray levels as a de facto standard.
Generally, it is desirable to expand halftone technology beyond the technologies supported by the PostScript specification. Further, it is also desirable to remove artifacts commonly found in halftone screens. Such artifacts include bands and optical jumps in gray levels. For example, half screens typically use the same dot pattern for over and over for a particular tone level, though other dot patterns for that tone level are possible. Thus, the halftone cells at many tone levels have a non-symmetrical arrangement that visually appear as bands. Further, the touch points between adjacent halftone cells are also non-symmetrical and appear as optical jumps in gray levels. These visual artifacts will be referred to hereinafter as “noise.”
Thus, there exists a need for a system and method that customizes a halftone screen to better reproduce certain image properties such as spatial frequencies and contrast. Further, a need also exists for a system and method that has the ability to eliminate certain noise problems commonly associated with screening such as touch point density growth and patterning.